JP4558633B2 - Wastewater treatment method containing fluoride ions - Google Patents

Description

The present invention relates to a method for reducing the content of fluorine ions contained in wastewater having a pH of 2.10 or less discharged from a semiconductor manufacturing factory. More specifically, the present invention relates to a method for reducing the content of harmful fluorine ions (hereinafter sometimes referred to simply as “fluorine”) contained in the wastewater below the wastewater standard, and further below the soil environmental standard.

In semiconductor manufacturing factories, a large amount of hydrofluoric acid is used as an etchant for silicon wafers. Since this hydrofluoric acid is washed with water or the like, the pH of the wastewater is as low as 4.0 or less and contains a high concentration of fluorine.
According to the drainage standards issued by the Environment Agency, the fluorine concentration in the test solution is set to 8 mg / L or less, and according to the soil environmental standards, it is set to 0.8 mg / L or less.
As a method for removing fluorine in fluorine-containing wastewater, a method has been proposed in which a calcium salt is added to fluorine-containing water to form poorly soluble calcium fluoride (see Patent Document 1).

Also, a magnesia-based adsorbent formed by firing a magnesium compound capable of forming magnesia in fluorine-containing wastewater (a metal oxide selected from kaolin, ferric oxide, calcium oxide, and alumina with magnesia as the main component) And a method of adsorbing and removing fluorine by adding at least one mixture of the above (see Patent Document 2).
However, the method of generating calcium fluoride by adding calcium salt to fluorine-containing wastewater described in Patent Document 1 has a high solubility of calcium fluoride of 0.0016 g / 100 g (18 ° C.), which clears the soil environmental standards. I can't. Moreover, since the magnesia type adsorbent described in Patent Document 2 is a mixture of magnesia and the metal, the performance of removing fluorine is insufficient.
JP-A-10-57969 Japanese Patent Laid-Open No. 57-197082

  An object of the present invention is to solve the problems of the prior art and to provide a treatment method for efficiently removing fluorine ions in low pH wastewater containing fluorine ions discharged from a semiconductor manufacturing factory.

  The present inventors have intensively studied a method for efficiently removing fluorine ions in low pH wastewater containing fluorine ions discharged from a semiconductor manufacturing factory. As a result, the present invention was completed by finding that specific magnesium oxide has a large fluorine adsorption ability.

  Further, by adding such magnesium oxide to the waste water containing high concentration of fluorine ions, the fluorine concentration in the treated liquid is 8 mg / L or less of the waste water standard, and further 0.8 mg / L or less of the soil environment standard. And found that it is possible. Furthermore, the present invention was completed by finding that magnesium hydroxide adsorbing fluorine ions does not elute fluorine ions in the re-elution test.

That is, according to the present invention, there is provided a method for reducing the fluorine ion concentration in wastewater having a pH of 2.10 or less containing fluorine ions discharged from a semiconductor manufacturing factory, which will be described below.
(1) Magnesium oxide having a BET specific surface area of 40 to 200 m ² / g obtained by baking magnesium hydroxide at 700 to 1,000 ° C. is added to wastewater having a pH of 2.10 or less containing fluorine ions. The method of removing the fluorine ion in the said waste_water | drain characterized by processing at the temperature of 10-25 degreeC, adding a flocculent, and carrying out solid-liquid separation.
(2) The method for removing fluorine ions according to (1) above, wherein the wastewater having a pH of 2.10 or less containing fluorine ions is wastewater discharged from a semiconductor manufacturing factory.
(3) The method for removing fluorine ions according to (1) above, wherein the fluorine ion concentration in the wastewater having a pH of 2.10 or less containing fluorine ions is 20 to 300 mg / L.
(4) The method for removing fluorine ions according to (1) above, wherein the magnesium oxide is obtained by baking magnesium hydroxide at 750 to 850 ° C. and has a BET specific surface area of 50 to 170 m ² / g.
(5) The magnesium oxide is obtained by baking magnesium hydroxide at 800 to 900 ° C., and has a BET specific surface area of 100 to 170 m ² / g.
(6) The method for removing fluorine ions according to the above (1), wherein the contact time between the waste water and the magnesium oxide is 5 minutes to 3 hours, preferably 15 minutes to 1 hour.
(7) The method for removing fluorine ions according to (1) above, wherein the liquid temperature at the time of contact between the waste water and the magnesium oxide is 10 to 25 ° C.
(8) The method for removing fluorine ions according to the above (1), wherein the pH of the treated waste water containing magnesium hydroxide particles formed is 9.5 to 11.0.
(9) The method for removing fluorine ions according to the above (1), wherein 0.1 to 10 parts by weight, preferably 0.2 to 5 parts by weight of the magnesium oxide is added per 100 parts by weight of the waste water.
(10) a flocculating agent, a method of removing the (1), wherein the fluorine ions allowed to magnesium oxide 0.01 parts by weight added pressure per 1 part by weight.

According to the method for removing fluorine ions by the method of the present invention, the following advantages and characteristics can be obtained.
(A) The separated fluoride ion-containing magnesium hydroxide particles do not re-elute fluoride ions in the Environmental Agency Notification No. 46 dissolution test.
(B) The magnesium hydroxide particles containing the separated fluorine ions have a magnesium hydroxide crystal pattern based on measurement by powder X-ray diffraction.
(C) The fluorine ion concentration in the waste water after treatment is 8 mg / L or less.
(D) The fluorine ion concentration in the wastewater is 0.8 mg / L or less in the soil environment standard after treatment.
(E) The treating agent (magnesium oxide) has a maximum adsorption capacity of 3 mmol / g which can reduce the fluorine ion concentration in the waste water after treatment to 8 mg / L or less.
(F) The treatment agent (magnesium oxide) has a maximum adsorption capacity of 1.6 mmol / g, which can reduce the fluorine ion concentration in the waste water after treatment to 0.8 mg / L or less.

According to the method of the present invention, the pH 2.10 in the following wastewater containing fluorine ions discharged from a semiconductor manufacturing factory, by adding a particular magnesium oxide, by forming a magnesium hydroxide particle, the processed waste water It is possible to easily reduce the fluorine ion concentration in the wastewater standard, or even below the soil environmental standard.

The embodiment of the method for reducing the fluorine ion concentration in waste water having a pH of 2.10 or less containing fluorine discharged from a semiconductor manufacturing plant according to the present invention is the method of adding specific magnesium oxide to the waste water and leaving the mixture stirred to leave fluorine. The magnesium hydroxide particles are formed, and the magnesium hydroxide is separated. That is, when magnesium oxide is introduced into water, magnesium hydroxide is produced by the following hydration reaction.
Hydration reaction of magnesium oxide: MgO + H ₂ O → Mg (OH) ₂
During this reaction, the adsorption mechanism was unknown, but it was found that fluorine ions can be adsorbed and removed.

  In the present invention, the fluorine ion concentration in the waste water to be treated is 20 to 300 mg / L. This is because the fluorine ion concentration in the wastewater discharged from the semiconductor manufacturing factory is 20 mg / L at the minimum and 300 mg / L at the maximum in our knowledge.

In the present invention, magnesium oxide is 700～1,000 ° C. Magnesium hydroxide, preferably 800 to 900 obtained by firing at ° C. and a BET specific surface area of 40 to 200 m ² / g, preferably 100~170m ² / g belongs to. For the firing of magnesium hydroxide, a rotary kiln is generally used. However, when firing at a temperature of less than 700 ° C. or more than 1,000 ° C., it becomes difficult to obtain magnesium oxide having a target BET specific surface area.

On the other hand, since the activity is low when the BET specific surface area of the magnesium oxide is less than 40 m ² / g, it has a long time for hydration in water (transition to magnesium hydroxide), and the fluorine ion adsorption capacity is small, which is disadvantageous. It is. The upper limit is not particularly limited, but it is difficult to obtain magnesium oxide having a specific surface area exceeding 200 m ² / g.

  In the present invention, the contact time between the waste water and the magnesium oxide is 5 minutes to 3 hours, preferably 15 minutes to 1 hour. A contact time of less than 5 minutes is disadvantageous because the amount of magnesium oxide transferred to magnesium hydroxide is small and the fluorine adsorption capacity is small. The upper limit is not particularly limited, but if it exceeds 3 hours, the processing time is long and disadvantageous.

  In this invention, the liquid temperature at the time of contact with this waste_water | drain and this magnesium oxide is 10-25 degreeC. When the liquid temperature is less than 10 ° C., it takes a long time to transfer magnesium oxide to magnesium hydroxide, which is disadvantageous. The upper limit is not particularly limited. However, since the liquid temperature is low in winter, it is economically disadvantageous because heat energy is required to obtain a temperature exceeding 25 ° C.

  In the present invention, the pH of the waste water from which the magnesium hydroxide particles containing fluorine ions are separated is 9.5 to 11.0. By adding a mineral acid such as sulfuric acid to this waste water, the pH of the waste water standard is easily reduced to 5 .6 to 8.6 is possible.

  In the present invention, the amount of magnesium oxide added is 0.1 to 10 parts by weight, preferably 0.2 to 5 parts by weight with respect to 100 parts by weight of the waste water. Although it does not restrict | limit in particular about an upper limit, it is because the fluorine density | concentration in this waste_water | drain can be made into a soil environmental standard value or less by addition of the said range.

In the present invention, a flocculant is added in order to facilitate solid-liquid separation of the treated magnesium hydroxide particles containing fluorine ions from the waste water.
As the flocculant, both commercially available inorganic flocculants and commercially available polymer flocculants are effective, and the amount added is preferably 0.01 to 0.5 parts by weight with respect to 1 part by weight of the magnesium oxide. It is.

  In the present invention, the separated fluoride ion-containing magnesium hydroxide particles meet the fluoride ion elution standard of the Environment Agency Notification No. 46 dissolution test. From this result, it is speculated that the fluoride ion adsorption mechanism of magnesium hydroxide produced by hydration of magnesium oxide is not due to the formation of magnesium fluoride (the solubility of magnesium fluoride is greater than the solubility of calcium fluoride) By much higher).

  In the present invention, the magnesium fluoride particles containing the separated fluorine ions have a magnesium hydroxide crystal pattern based on measurement by powder X-ray diffraction.

  According to a preferred embodiment of the present invention, the fluorine ion concentration in the fluorine ion-containing waste water can be set to 8 ppm or less of the drainage standard value, and further to 0.8 ppm or less of the soil environment standard value. Moreover, the fluorine ion adsorption capacity of the magnesium oxide when achieving these is 3 mmol / g or more and 1.6 mmol / g or more, respectively.

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples.
In addition, the measurement of various ion concentrations in the waste water is JIS K 0102 34.1 for fluorine ions, ion chromatography for chlorine ions and nitrate ions, and others (aluminum ions, magnesium ions, calcium ions, sodium ions, sulfate ions). Were analyzed by ICP respectively.
X-ray diffraction was measured with Cu-Kα using RINP2200V manufactured by Rigaku Corporation.

Example 1
1 g (0.5 parts by weight) of magnesium oxide “Kyowa Mag 30: BET surface area = 48 m ² / g” manufactured by Kyowa Chemical Industry Co., Ltd. is added to 200 mL of waste water discharged from a semiconductor manufacturing plant containing ions shown in Table 1. The mixture was stirred for 30 minutes using a magnetic stirrer (the liquid temperature during stirring was 23.3 ° C.). Thereafter, the fluoride ion concentration of the supernatant liquid separated into solid and liquid was 6.72 mg / L, which was in conformity with the drainage standard. The pH of the solution was 9.92.

  In addition, the fluoride ion adsorption capacity of the magnesium oxide under this condition was 0.92 mmol / g.

Example 2
Wastewater 200mL discharged from the semiconductor manufacturing factory comprising ions shown in Table 1, manufactured by Kyowa Chemical Industry Co., Ltd. Magnesium oxide "Kyowa Mag 30: BET surface area = ^48m 2 / g" and 0.65 g (0.33 parts by weight) The mixture was added and stirred for 10 hours using a magnetic stirrer (the liquid temperature during stirring was 23.1 ° C.). After that, the fluoride ion concentration of the supernatant liquid obtained by solid-liquid separation was 0.4 mg / L, which conformed to the soil environmental standards. The pH of the solution was 10.35.
In addition, the fluoride ion adsorption capacity of the magnesium oxide under these conditions was 1.51 mmol / g.

Example 3
0.5 g (0.25 part by weight) of magnesium oxide “Kyowa Mag 100: BET surface area = 85 m ² / g” manufactured by Kyowa Chemical Industry Co., Ltd. was added to 200 mL of waste water discharged from a semiconductor manufacturing factory containing ions shown in Table 1. The mixture was added and stirred for 30 minutes using a magnetic stirrer (the liquid temperature during stirring was 23.4 ° C.). After that, the fluorine ion concentration of the supernatant liquid separated into solid and liquid was 1.95 mg / L, which met the drainage standard. The pH of the solution was 10.32.
In addition, the fluoride ion adsorption capacity of the magnesium oxide under these conditions was 1.94 mmol / g.

Example 4
Example 3 was the same as Example 3 except that the amount of magnesium oxide added was 0.7 g (0.35 parts by weight). As a result, the fluorine ion concentration of the supernatant liquid obtained by solid-liquid separation was 0.2 mg / L, which conformed to the soil environmental standards. The pH of the solution was 10.30.
In addition, the fluoride ion adsorption capacity of the magnesium oxide under these conditions was 1.41 mmol / g.

Example 5
0.3 g (0.15 parts by weight) of magnesium oxide “Kyowa Mag 150: BET surface area = 146 m ² / g” manufactured by Kyowa Chemical Industry Co., Ltd. was discharged into 200 mL of waste water discharged from a semiconductor manufacturing plant containing ions shown in Table 1. The mixture was added and stirred for 30 minutes using a magnetic stirrer (the liquid temperature during stirring was 23.5 ° C.). After that, the fluoride ion concentration of the supernatant liquid separated into solid and liquid was 3.1 mg / L, which was in conformity with the drainage standard. The pH of the solution was 10.49.
In addition, the fluoride ion adsorption capacity of the magnesium oxide under these conditions was 3.2 mmol / g.

Example 6
Example 5 was the same as Example 5 except that the amount of magnesium oxide added was 0.6 g (0.3 parts by weight). As a result, the fluorine ion concentration of the supernatant liquid obtained by solid-liquid separation was 0.22 mg / L, which conformed to the soil environmental standards. The pH of the solution was 10.45.
In addition, the fluoride ion adsorption capacity of the magnesium oxide under these conditions was 1.64 mmol / g.

Example 7
200 mL of waste water discharged from a semiconductor manufacturing plant containing ions shown in Table 1 was adjusted to a liquid temperature of 10 to 12 ° C., and magnesium oxide “Kyowa Mag 150: BET surface area = 146 m ² / g” manufactured by Kyowa Chemical Industry Co., Ltd. 0.0 g (0.5 part by weight) was added, and the mixture was stirred for 30 minutes using a magnetic stirrer (during this time, the liquid temperature was adjusted to 10 to 12 ° C. throughout). After that, the fluoride ion concentration of the supernatant obtained by solid-liquid separation was 0.31 mg / L, which conformed to the soil environmental standards. The pH of the solution was 10.40.
In addition, the fluoride ion adsorption capacity of the magnesium oxide under these conditions was 0.99 mmol / g.

Example 8
To 3000 mL of waste water discharged from a semiconductor manufacturing plant containing ions shown in Table 1, 9.0 g (0.3 parts by weight) of magnesium oxide “Kyowa Mag 150: BET surface area = 146 m ² / g” manufactured by Kyowa Chemical Industry Co., Ltd. The mixture was added and stirred for 1 hour using a chemistor. Later, when 0.3 g of flocculant Skyclean S manufactured by Mikuni Ecosystem Co., Ltd. (3.3 parts by weight with respect to 1 part by weight of magnesium hydroxide) was added, a floc with good sedimentation was formed, and stirring was performed for 20 seconds. The solid-liquid separation was complete. Thereafter, stirring was stopped, and the fluoride ion concentration in the liquid adjusted to pH 7.5 by adding sulfuric acid to the supernatant liquid separated into solid and liquid was 0.3 mg / L, which passed the soil environmental standard. In addition, the fluoride ion adsorption capacity of the magnesium oxide under these conditions was 1.64 mmol / g.
Further, when the solid separated into solid and liquid was dried, the yield was 12.8 g, which almost coincided with the theoretical magnesium hydroxide amount of 13.0 g obtained by hydrating 9 g of magnesium oxide. Further, the X-ray diffraction pattern of this dried product showed a crystal pattern of magnesium hydroxide as shown in FIG.

Example 9
In Example 8, using the solid material obtained by solid-liquid separation, the results of carrying out fluorine ion-containing magnesium hydroxide particles (magnesium hydroxide particles adsorbing fluorine ions) according to the Environmental Agency Notification No. 46 dissolution test The eluted fluorine ion concentration was less than 0.1 mg / L, which passed the soil environmental standards.

Example 10
To 200 mL of waste water discharged from a semiconductor manufacturing plant containing ions shown in Table 2, 2.5 g (1.25 parts by weight) of magnesium oxide “Kyowa Mag 150: BET surface area = 146 m ² / g” manufactured by Kyowa Chemical Industry Co., Ltd. The mixture was added and stirred for 30 minutes using a magnetic stirrer (the liquid temperature during stirring was 23.8 ° C.). Thereafter, the fluoride ion concentration of the supernatant liquid obtained by solid-liquid separation was 0.25 mg / L, which met the drainage standard. The pH of the solution was 10.38.
In addition, the fluoride ion adsorption capacity of the magnesium oxide under this condition was 1.24 mmol / g.

Comparative Example 1
1 g of magnesium oxide (Kyowa Mag 150: BET surface area = 146 m ² / g) manufactured by Kyowa Chemical Industry Co., Ltd. was added to 200 mL of pH 8.20 model wastewater (sodium fluoride aqueous solution) adjusted to a fluorine ion concentration of 98.2 mg / L. 0.5 parts by weight) was added and stirred for 30 minutes using a magnetic stirrer (the liquid temperature during stirring was 22.2 ° C.). Thereafter, the F ion concentration in the supernatant obtained by solid-liquid separation was 25.5 mg / L, which was incompatible with the drainage standard. The pH of the solution was 11.85.
In addition, the fluoride ion adsorption capacity of the magnesium oxide under these conditions was 0.77 mmol / g.

Comparative Example 2
In Comparative Example 1, the same treatment as in Comparative Example 1 was performed except that the stirring time in the magnetic stirrer was changed to 24 hours. As a result, the fluorine ion concentration was 11.3 mg / L, which was incompatible with the wastewater standard.
In addition, the fluoride ion adsorption capacity of the magnesium oxide under these conditions was 0.92 mmol / g.

From Examples 1 to 10, magnesium hydroxide particles containing fluorine ions are produced by introducing magnesium oxide into fluorine ion-containing wastewater discharged from a semiconductor manufacturing factory and generating magnesium hydroxide particles by rehydration. It can be seen that the fluorine ions in the waste water can be efficiently removed. Furthermore, the maximum adsorption capacity that can be reduced to a fluorine concentration of 8 mg / L or less based on wastewater is 3 mmol / g or more, and the maximum adsorption capacity that can be set to 0.8 mg / L or less based on soil environment is 1.6. It can also be seen that it is at least mmol / g.
From this adsorption capacity, the amount of magnesium oxide to be added can be determined by calculation regardless of the fluorine ion concentration in the waste water.
Furthermore, it can be seen that magnesium hydroxide adsorbed with fluoride ions does not elute fluoride ions in the dissolution test, and therefore can be easily disposed of at low cost.
Moreover, it turns out that this magnesium oxide cannot remove a fluorine ion efficiently unless it is low pH waste_water | drain by Comparative Examples 1-2.

FIG. 9 shows an X-ray diffraction pattern of magnesium hydroxide particles formed in Example 8 of the present invention.

Claims (4)

Magnesium oxide obtained by baking magnesium hydroxide at 700 to 1,000 ° C. and having a BET specific surface area of 40 to 200 m ² / g is added to wastewater having a pH of 2.10 or less containing fluorine ions, A method for removing fluorine ions in the waste water, characterized by treating at a temperature of 25 ° C. and adding a flocculant to perform solid-liquid separation. The method for removing fluorine ions according to claim 1, wherein the wastewater having a pH of 2.10 or less containing fluorine ions is wastewater discharged from a semiconductor manufacturing factory. The method for removing fluorine ions according to claim 1, wherein the fluorine ion concentration in the wastewater having a pH of 2.10 or less containing fluorine ions is 20 to 300 mg / L. The method for removing fluorine ions according to claim 1, wherein the magnesium oxide is obtained by baking magnesium hydroxide at 750 to 850 ° C and has a BET specific surface area of 50 to 170 m ² / g.

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